drm/exynos: dsi: restore support for drm bridge

This patch fixes issue introduced by commit
cf67cc9a ("drm/exynos: remove struct
exynos_drm_display"), which removed assigning of drm bridge to drm
encoder. Lack of it caused that no bridge callbacks were called on
encoder enable/disable actions.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>